RFID tags are becoming a well established method for tracking materials during shipping and storage. In many applications they replace the printed bar code labels on items because they do not require a close proximity for the automatic reader. In the usual tag interrogation process, a reader or interrogator projects energy towards the item to be tracked, with the energy picked up by an antenna on the tag and transferred to the integrated circuits utilized to transmit specific item information back through the antenna to the reader.
In most cases the reader employs a dipole antenna, which is linear in polarization. The tag itself usually is provided with a linearly polarized antenna such as a loop or dipole and may have an arbitrary orientation relative to the ground. Since the linear polarization makes the tag directional, this presents problems when transmitting from the reader to the tag and vice versa. The polarization may be rotated 90 degrees from the reader antenna, or the dipole radiation may have a null in the radiation pattern pointed toward the reader. It would therefore be desirable to provide a tag with gain in all directions to be able to guarantee communications between the reader and the tag.
More particularly, most RFID tags employ a linearly polarized antenna. It will be appreciated that the orientation of the tag is not known, which means that there will not be optimal efficiency in transferring the energy from the reader to the integrated circuits in the tag or for that matter optimally transmitting the information from the tag back to the reader.
RFID tags come in both active and passive forms. In the passive form, the tag is parasitically powered by the energy from the reader or interrogator. Because of the diodes within the rectennas utilized in the tags, there is a threshold level that must be exceeded so that the integrated circuits in the tag can be powered.
In an extreme example, if the linearly polarized antenna for the reader is orthogonal to the linearly polarized antenna of the tag, then no energy will transfer from the reader to the tag. Not only will communication between the two be impossible, it will not be possible to parasitically power the tag.
Since the orientation of the tag relative to the reader is not easily controlled, it is important to be able to have an omnidirectional antenna located on the tag so that energy is transferred efficiently between the reader and the tag.